Operational transconductance amplifiers (OTAs) are useful in applications such as telecommunications. For example, an OTA-C filter, built using an OTA with a capacitor, is a substitute for a more expensive switched capacitor filter. OTAs receive an input voltage and in response provide a current whose magnitude is defined by the transconductance of the OTA. Transconductance, g.sub.m, is defined as the change in output current divided by the change in input voltage, or EQU g.sub.m =.DELTA.I/.DELTA.V (1)
where .DELTA.I is the change in output current, and .DELTA.V is the change in input voltage. In another form, the input voltage and the output current may be differential. OTAs may have different values for g.sub.m suited for particular applications. For example, it is desirable for low frequency filters to have low values for g.sub.m, in the range of 360-800 nanoamperes/volt, so that the accompanying capacitor need not be too large. Large-valued capacitors cannot be implemented on monolithic integrated circuits.
An ideal OTA has infinite input impedance and infinite output impedance. Linearity of the OTA can be measured by the ratio of signal to total harmonic distortion (THD), in decibels or percent. Dynamic range can be measured by the range of input voltages at a given power supply voltage. In addition, it is preferable for OTAs to operate from commonly available 5-volt power supplies. Real OTAs, however, fall short of the ideal characteristics. The performance of different known OTA designs is summarized by Van Peteghem et al. in "A Very-Linear CMOS Transconductance Stage for OTA-C Filters", IEEE 1989 Custom Integrated Circuits Conference. As discussed by Van Peteghem et al., known OTAs trade off linearity, dynamic range, and operating voltage. This tradeoff is to some extent inherent in transistor technology. MOS transistors and bipolar transistors both exhibit nonlinear input/output characteristics. MOS transistors amplify an output current in response to an input voltage defined by the equation EQU I.sub.DS =(K'W/2L)(V.sub.GS -V.sub.T).sup.2 ( 2)
where I.sub.DS is the drain-to-source current, K' is the Boltzmann constant, W is the gate width of the transistor, L is the gate length of the transistor, V.sub.GS is the gate-to-source voltage of the transistor, and V.sub.T is the transistor threshold. Thus the output current of a MOS transistor is related to the input voltage by a square law. For small changes in V.sub.GS, where V.sub.GS is close to V.sub.T, the characteristic is approximately linear, and this feature can be used to improve the linearity of the OTA. However, requiring only small variations in V.sub.GS limits the dynamic range.
One method to improve the linearity for MOS transistor OTAs is source degeneration. In MOS transistor OTAs, typically two transistors receiving a differential input signal on their respective gates, have their sources connected together and selectively divert a current driven by a constant-current sink. When source degeneration is used, resistors are connected between the sources of the input transistors and the current sink. The resistors decrease the effect of a change in the differential voltage on the gates of the transistors to the change in the output current, because most of the voltage drop occurs across the resistors. Source degeneration improves linearity and lowers g.sub.m. To implement source degeneration, resistors or MOS transistors operated in the linear range may be used. However, integrated circuit resistors commonly implemented as diffusions in the semiconductor body introduce nonlinearities. Furthermore, use of resistors and linear region MOS transistors worsens dynamic range.